1. Field of the Invention
The present invention relates to rotary cutting tools in general, and to rotary cutting tools having chip-breaking features in particular.
2. Background Information
Rotary cutting tools such as end mills typically have a cylindrical configuration that includes a shank portion and a cutting portion. The cutting portion contains a plurality of helically disposed cutting blades that extend from a first end (i.e., the “shank end”) of the cutting portion adjacent the shank portion, toward the opposite end (i.e., the “free end”) of the cutting portion. In some embodiments, the cutting edges of the helical blades are disposed along a substantially constant radius with respect to the longitudinal axis of the tool. In other embodiments, generally referred to as “tapered” cutting tools, the cutting portion is substantially frustoconical in shape; i.e., the cutting edge of each blade has a constantly decreasing radius with respect to the longitudinal axis of the tool as the cutting edge extends from the shank end of the cutting portion to the free end. The cutting edges of the blades in a tapered rotary cutting tool are at the same radius from the longitudinal axis of the tool in any plane through the cutting portion and perpendicular to the longitudinal axis of the tool. In still other end mill embodiments, generally referred to as “straight-fluted” rotary cutting tools, the cutting edges of the blades extend parallel to the longitudinal axis of the tool.
There are several inherent problems in the use of any of the conventional rotary cutting tools described above. Generally, these problems manifest themselves in excessive wear and relatively poor cutting actions, or both, due to the fact that the entire length of the cutting edge may be applied to the workpiece at the same time, and due to the fact that continuous chips are produced which are not adequately removed from the work area. There have been many attempts to improve the cutting action and decrease the wear in such tools, and these attempts usually involve the use of so called “chip breakers” in the form of relatively deep notches cut transversely into the cutting blade at spaced intervals, or some similar form of providing an interrupted cutting edge along each blade. One such form as hereinabove referred to is a “chip breaker” described in U.S. Pat. No. 2,855,657, issued Oct. 14, 1958, which discloses that the cutting edge of each blade is provided at uniformly spaced intervals along each helical blade with notches of substantial depth which are ground therein for the purpose of interrupting the chips produced by the plurality of helically disposed parallel cutting blades. It is further noted that the notches in successive blades circumferentially of the tool are slightly axially offset, this effect preferably being obtained by grinding such notches in a low pitch helical path circumferentially of the tool. The '657 patent further discloses a shallow bevel merging with each such notch and formed on one side of the notch of one blade and on an opposite side of such notch of the next adjacent blade so that they alternate first to one side and then to the other. The purpose of such arrangement is to balance out the endwise thrust on the tool and to maintain the torque more nearly centered. This attempted solution to the aforementioned problems has met with some success in improvement of tool life and in facilitating chip removal from the work area, although such success is due primarily to the fact that smaller, discontinuous chips are produced which may be more readily removed from the work area. In other words, the solutions have been directed primarily to a form of the chip produced, rather than removal of the chips from the work area. Also, in the construction as suggested by the '657 patent wherein the helical cutting edges are interrupted axially of the body by one or more helical grooves which spiral about the body at either the same or different pitches as the flutes, but in opposite directions, that is, the flutes spiral in the right-hand direction, whereas the grooves spiral in a left-hand direction, the grooves then are disposed in a direction which impedes chip removal. That is, a chip at the leading edge of a cutting tool defined by a groove normally tends to move towards the shank end of the cutter along the helix of the flute, but if the chip enters the groove, it is urged toward the cutting end of the cutter. As a result, the pressure relief advantages caused by grooves are dissipated to some extent, the tool operates at a higher temperature, and the smoothness of the cut suffers.
Several attempts have been made in the prior art to solve this problem. For example, U.S. Pat. No. 3,548,476 discloses a cutter having a plurality of helical flutes of uniform length and depth which form a plurality of helical cutting edges circumferentially spaced from one another by the flutes and which spiral about the axis of the body in the same direction at the same pitch as the flutes. Each of the cutting edges extends radially of the body and merges smoothly with its associated cutting edge which spirals in a right-hand direction about the axis of the body at a predetermined angle. These cutting edges are interrupted longitudinally at spaced intervals by a plurality of uniformly spaced notches. The notches are formed by a groove which spirals about the axis of the body in the same direction (right-hand helix for both flutes and notch groove) of the flutes but at a steeper pitch. The helix on which the notches are formed and the width of the notches in the blades is selected so that the circumferentially adjacent blades on successive cutting edges are offset axially from one another by an amount such that each circumferentially successive blade has a portion which follows a portion of a groove in a preceding cutting edge. The amount of offset between successive blades should be such that, for each complete revolution of the body, the blade trailing any given notch will more than offset the length of the notch. A cutting tool construction in accordance with the disclosure of the '476 patent defines advantages in that the formation of discontinuous chips facilitates chip removal and the spiraling of the notches in the same direction as the flutes further facilitates chip removal. Moreover the discontinuous cutting blades result in less drag or resistance to rotation of the tool when it is in operative engagement with a workpiece, thereby reducing deflection of the tool and permitting cutting of the workpiece to closer tolerances than would otherwise be possible, enabling the tool to operate at a relatively low temperature and thereby increasing tool life. The specific problems inherent in the cutting tool described in the '476 patent, however, is that the notches which form the groove which spirals about the axis of the body in the same direction as the flutes generate a strong longitudinal force during a cut and, eventually, as the tool dulls this force will become sufficient to pull the cutting tool from its workholding device.
U.S. Pat. No. 4,212,568, issued to Minicozzi, is also directed to the problem of facilitating chip removal from the work area while improving the tool life. Minicozzi discloses that the leading edge and trailing edge of each blade is interrupted by a plurality of relatively shallow transverse depressions of relatively large radius arcuate cross-section resulting in cutting edges with a variable rake angle which tends to reduce tool wear. Further, the sinusoidally undulating surface of the cutting face of each blade gives rise to a rake angle at each cutting edge which varies continuously along the length of the cutting edge, facilitating the formation of relatively small chips. When these chips move through the space where the cutting and trailing face surfaces are convex they tend to squeeze at these areas and tend to spring away from the workpiece and the cutting tool when free to do so, in this way facilitating chip removal. The cutting tool disclosed in Minicozzi is time consuming to manufacture and, therefore, expensive. Additionally, its chip removing feature provides little improvement to the feed and speed characteristics of the cut.
U.S. Pat. No. 4,285,618, issued to Shanley, Jr., recognizes the need for rapid cutting and the requirement of smooth dimension finishing qualities simultaneously. Accordingly, Shanley, Jr. discloses a conventional cutting tool of a hard metal body whose cutting portion surface is formed into a plurality of blades separated by flutes, each blade having a leading side, a cutting edge on the leading side, a land, and a trailing side of face; at least two of the blades having at least one smooth segment, wherein the land and cutting edge are even and unbroken, and at least one serrated segment formed with crests that are flat, round, or sinusoidal, wherein the land consists essentially of a row of cutting blades, adjacent blades in the lands being separated from each other by a transverse groove in the blade. The smooth end serrated segments are located in staggered positions from blade to blade, so that in the course of one complete revolution of the cutter, each point along a surface being formed by the cutter will be contacted by at least one smooth segment and at least one serrated segment. The serrated segments are arranged in such a pattern from blade to blade that continuous, imaginary lines passing across each blade at precisely the midpoint of each serrated segment would define a helix of uniform angle around the cutting section measured from a line which is parallel to the shank section. Shanley, Jr. further discloses that the helix may be right-handed or left-handed such that the helix of the serrated segments is the same as the helix of the blades on the cutting portion of the tool. Generally, the cutter will be capable of faster metal removal if the lay of the helical serration pattern is opposite that of the helical pattern of the blades. Shanley Jr. discloses that the blades in the serrated segments may be formed with flat, rounded or sinusoidal crests. This type of blade formation, however, will result in drag and galling of the leading edge of the cutting blade since the leading edge of the cutting tool has a negative rake angle.
U.S. Pat. No. 4,497,600, issued to Kishimoto, discloses an end mill wherein the shape and the arrangement of the notch required for the blade can be freely selected without using thread cutting. Kishimoto discloses a cutting tool wherein along the whole circumferential surface of each blade, notches are machined at prescribed intervals, extending transverse to the length of each blade. Each notch is shifted slightly along the blade toward the tool end or the shank end relative to a corresponding notch on the preceding blade. When X is taken as the width of the blade surface between notches, Y as the width of the notch, and Z as the amount of shift of a notch relative to the corresponding notch in the adjacent preceding blade, the shift (Z) of the notch is at least equal to (X+Y)/(N) (where N is the number of blades). In the case of shifting the notch toward the tool end, the inclination of the notch on its own axis is toward the shank end and when shifting the notch toward the shank end, the inclination of the notch on its own axis is toward the tool end. Since the notches are formed at the prescribed intervals and with a shift of a prescribed amount, rather than being formed along a helical path as in a screw thread, the side clearance of the notch is not limited by the number of blades. Since the direction of the side clearance is varied by the direction of the shift of the notches relative to the preceding and succeeding blades, the cutting by the main blade is formed with an effect of elevating the cutting performance. The angle of the notch and the shape of the notch can be freely selected depending on the material to be cut. In the conventional tool in which the notch is formed along a helix, such as a thread, these values are fixed. The disclosure of Kishimoto results in a notch in a blade which will have a negative rake angle on the leading or trailing edge of the blade form. The negative rake angle causes drag and galling of the metal at the cutting edge of the blade as well as heat build up and higher wear or shorter life of the cutting tool.
What is needed is a rotary cutting tool that overcomes the shortcomings of the prior art.